1. Field of the Invention
The present invention relates generally to the field of optical waveguide fibers, and more particularly to optical waveguide fiber preforms and methods of making optical waveguide fiber preforms from which low water peak optical waveguide fibers are formed.
2. Technical Background
Generally speaking, a significant goal of the telecommunications industry is to transmit greater amounts of information, over longer distances, in shorter periods of time. Typically, as the number of systems users and frequency of system use increase, demand for system resources increases as well. One way of meeting this demand is by increasing the bandwidth of the medium used to carry this information over long distances. In optical telecommunications systems, the demand for optical waveguide fibers having increased bandwidth is particularly high.
In recent years, significant advancements have been made in the manufacture of optical waveguide fiber, which in turn have increased the usable light carrying capacity of the fiber. However, as is well known, electromagnetic radiation traveling through an optical waveguide fiber is subject to attenuation or loss due to several mechanisms. Although some of these mechanisms can not be reduced, others have been eliminated, or at least substantially reduced. A particularly problematic component of optical fiber attenuation is the attenuation due to absorption by the optical waveguide fiber of impurities present in the light guiding region of the fiber. Particularly troublesome is the attenuation caused by the hydroxyl radical (OH), which can be formed in the optical waveguide fiber when a source of hydrogen is present in the fiber material, or when hydrogen available from several sources during the fiber manufacturing process diffuses into the glass. Generally speaking, the hydrogen bonds with the oxygen available in the SiO2 and/or GeO2 and/or other oxygen containing compound in the glass matrix to form the OH and/or OH2 bonds referred to generally as xe2x80x9cwaterxe2x80x9d. The attenuation increase due to OH or water in the glass can be as high as about 0.5 to 1.0 dB/km, with the attenuation peak generally occupying the 1380 nm window. As used herein, the phrase, xe2x80x9c1380 nm windowxe2x80x9d is defined as the range of wavelengths between about 1330 nm to about 1470 nm. The attenuation peak, generally referred to as the water peak, has prevented usable electromagnetic transmission in the 1380 nm window.
Until recently, telecommunications systems avoided the water peak residing in the 1380 nm window by operating in the 1310 nm window and/or the 1550 nm window, among others. With the advent of wavelength division multiplexing (xe2x80x9cWDMxe2x80x9d) and advancements in amplifier technology, which enable telecommunications systems to operate over broad wavelength ranges, it is now likely that all wavelengths between about 1300 nm and about 1650 nm will be used for data transfer in optical telecommunications systems. Removing the water peak from optical waveguide fiber used with such systems is an important aspect of enabling system operation over this entire range.
One aspect of the present invention relates to a method of fabricating a cylindrical glass body for use in manufacturing optical waveguide fiber. The method includes the steps of chemically reacting at least some of the constituents of a moving fluid mixture that includes at least one glass forming precursor compound in an oxidizing medium to form a silica-based reaction product. At least a portion of the reaction product, which includes hydrogen bonded to oxygen, is collected or deposited to form a porous body. A centerline hole extending axially through the porous body is formed during the deposition process, for example, by depositing the reaction product on a substrate, and thereafter removing the substrate. The porous body is dried and consolidated to form a glass preform, and the centerline hole is closed under conditions suitable to make an optical fiber having optical attenuation of less than about 0.35 dB/km at a wavelength of 1380 nm. Preferably, the fiber exhibits optical attenuation less than 0.31 dB/km at a wavelength of 1380 nm.
In another aspect, the present invention relates to a cylindrical glass body for use in manufacturing optical waveguide fiber that is made by the method described above.
A further aspect of the present invention is directed to an optical waveguide fiber. The optical waveguide fiber includes a silica containing core glass, at least a portion of which includes hydrogen bonded to oxygen. The silica containing core glass further includes a centerline region, at least a portion of which includes a dopant, and is formed by closing a centerline hole of a preform. A cladding glass surrounds the silica containing core glass so that the optical waveguide fiber exhibits an optical attenuation of less than about 0.31 dB/km at a wavelength of about 1380 nm
In yet another aspect, the present invention is directed to an optical fiber communication system. The system includes a transmitter, a receiver, and an optical fiber for communicating an optical signal between the transmitter and the receiver. The optical fiber includes a silica containing core glass, at least a portion of which contains hydrogen bonded to oxygen, having a dopant containing centerline region formed by closing the centerline hole of a preform. The optical fiber further includes a cladding glass surrounding the silica containing core glass. Preferably, such an optical fiber exhibits an attenuation of less than about 0.31 dB/km at a wavelength of about 1380 nm.
A still further aspect of the present invention is directed to a method of fabricating a cylindrical glass body for use in manufacturing optical waveguide fiber. The method includes the steps of chemically reacting at least some of the constituents of a moving fluid mixture that includes at least one glass forming precursor compound in an oxidizing medium to form a silica-based reaction product. At least a portion of the reaction product, which includes hydrogen bonded to oxygen, is collected or deposited to form a porous body. A centerline hole extending axially through the porous body is formed during the deposition process, for example, by depositing the reaction product on a substrate, and thereafter removing the substrate. The porous body is dried and consolidated to form a glass preform having a centerline hole, which is subsequently closed. The drying, consolidating, and closing steps are performed under conditions suitable to result in a solid glass body including a centerline region having a weighted average OH content of less than about 1 ppb.
An additional aspect of the present invention relates to a plug for use in sealing the centerline hole of a soot blank used to manufacture optical waveguide fiber. The silica containing glass plug has an OH content of less than about 5 ppm by weight and is preferably chemically dried such that it has an OH content of less than about 1 ppb by weight.
The method of the present invention results in a number of advantages over other methods known in the art. Conventionally, optical waveguide fiber blanks made by an outside vapor deposition (OVD) process are consolidated in a chlorine containing atmosphere to chemically dry the blank and thus form a consolidated glass preform having a centerline hole extending axially therethrough. The core glass preform is then typically positioned within a redraw furnace and heated to a temperature sufficient to facilitate redrawing or stretching of the core preform into a smaller diameter cylindrical glass body or core cane. During the redraw operation, the centerline hole of the core blank is closed by, for example, applying vacuum (e.g., pressure of about 200 mTORR or less) along the centerline hole. The reduction in pressure within the centerline hole ensures complete closure of the centerline hole such that the core cane has a solid centerline region extending axially therethrough.
After the redraw step, the resulting core cane is typically overclad with a layer of cladding soot by depositing a cladding soot, e.g. via an OVD process. Once covered with sufficient cladding soot, the resulting soot overclad core cane is chemically dried and consolidated to form an optical fiber preform, which can thereafter be drawn into an optical waveguide fiber. Despite the chemical drying and consolidation steps, such optical waveguide fibers have been found to exhibit a relatively high level of attenuation measured at approximately 1380 nm. Because telecommunications systems presently in use today do not operate at or in the immediate vicinity of 1380 nm, this shortcoming has been largely overlooked. With recent advancements made in WDM, amplifier technology, and laser sources, however, eliminating the water peak measured at 1380 nm has become a priority. The water peak is largely a result of water being trapped in the glass during the fiber manufacture process. In the case of the OVD process, it is believed that a large portion of the water is trapped within the centerline region of the core cane prior to or during closure of the centerline hole. Although the blanks are chemically dried and sintered during consolidation, it has been found that the region of glass surrounding and defining the centerline hole is being rewet after drying. Most commonly, such rewetting occurs through the physisorption of water (OH2) and/or chemisorption of water (Beta OH) upon exposure of the centerline hole to an atmosphere that includes a hydrogen containing compound, such as, but not limited to water (H2O) following consolidation.
A principle advantage of the method of the present invention is that it greatly reduces the amount of water trapped within the centerline region of the core cane. Accordingly, optical waveguide fiber made from such a core cane exhibits a much smaller water peak at 1380 nm, and in the 1380 nm window as a whole, and therefor exhibits lower optical attenuation in the 1380 nm window than optical waveguide fiber manufactured in accordance with standard methods from preforms manufactured by the OVD process.
An additional advantage of the method and cylindrical glass body of the present invention is that optical waveguide fiber manufactured from such cylindrical glass bodies can now operate at any selected wavelength over a range of wavelengths from about 1300 nm to about 1680 nm without undo optical attenuation. More specifically, such fibers exhibit less than about 0.35 dB/km, and preferably less than about 0.31 dB/km at each wavelength within a wavelength range from about 1300 nm to about 1680 nm. Moreover, the method of the present invention is also economical to implement and can be practiced without the production of additional environmentally unfriendly waste products.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.